nir: Strengthen assertion that 'out' is nonnull.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

src/compiler/nir/nir_lower_clamp_color_outputs.c

@@ -79,7 +79,7 @@ lower_intrinsic(lower_state *state, nir_intrinsic_instr *intr)
             break;
          }
       }
-      assert(out);
+      assume(out);
       break;
    default:
       return;